Power transmission units for transmission components are known in a plurality of versions from the existing art. These normally include an input and an output, between which a hydrodynamic component for hydrodynamic power transmission is situated, comprising at least a pump wheel and a turbine wheel, which together form a working chamber which is filled with or may be filled with process fluid; in the case of versions such as torque converters there are also a guide wheel and a device in the form of a lockup clutch to bypass the hydrodynamic power branch. The lockup clutch also comprises at least a first friction surface arrangement having at least one element carrying a friction surface, and a second friction surface arrangement having at least one element carrying a friction surface, which may be brought into operative connection with each other by means of an actuating mechanism. The function of the lockup clutch here is to create a coupling between the input or the connection between the input and the pump wheel and the turbine wheel. In the simplest case, the actuating mechanism comprises an axially movable piston which presses the individual friction surface arrangements against each other. The power transmission device is designed as a two-channel or three-channel system, depending on the operating principle of the lockup clutch. At the same time, the flow through the hydrodynamic component may be centrifugal or centripetal, it also being possible for the directions of flow to change depending on the mode of operation. In the case of hydrodynamic torque converters, the process fluid remains in the converter even during bypassing. A coolant circuit forms an external converter circuit through the connections on the converter and the intermediate space between actuating mechanism and housing. The guidance takes place via channels integrated into the clutch arrangement, in particular between the friction elements that can be brought into operative connection with each other, radially outwardly to the outer circumference, viewed from the axis of rotation, and after that radially inwardly on the hydrodynamic component, or vice versa. In the latter cases, the lockup clutch is operated by the pressure in the hydrodynamic component; that is, the pressure present outside of the hydrodynamic rotational speed/torque converter or the hydrodynamic clutch is used to operate the piston element. Such designs are described for example in published patent DE 199 20 542 A1. The latter reveals a power transmission device having a fluid clutch or a hydrodynamic rotational speed/torque converter and an integrated damping system. In this design the actuating mechanism is either supported so that it is movable in the axial direction on the connection between pump wheel and input of the power transmission device and guided so that it is sealed against the connection between input of the power transmission unit and pump wheel. Between the inner wall of the connecting housing, between the input of the power transmission unit and the pump wheel, a pressure chamber functioning as an actuating chamber is formed, which serves to apply pressure to the piston of the actuating mechanism independent of the pressure in the hydrodynamic component. To that end, along with the connections to the converter or hydrodynamic component, normally in the pump neck or support shaft for the guide wheel and a connection to create and maintain an external circuit, in particular for cooling purposes, an additional connection is provided, through which the pressure chamber between actuating mechanism and housing may be charged with pressure medium. The system is therefore referred to as a three-channel system. The linking of the actuating element and the design of the lockup clutch turn out to be very complex here however, in particular due to the large number of existing components. The lockup clutch is designed as a lamellar clutch, and because of the separate pressure chamber the clutch elements which are connected at least indirectly to the pump wheel are designed as outer lamellae, while the second clutch elements connected to the output of the power transmission device, in particular in this case to the device for damping vibrations, are designed as inner lamellae. The actuating mechanism works here in the axial direction against the lamellae, for which reason an additional stop is also provided here in the axial direction to brace the actuating mechanism in the axial direction during actuation, which is connected in a rotationally fixed connection to the connection between the input of the power transmission device and the pump wheel. The lockup clutch itself can only be designed as a multiple-plate clutch, in particular a lamellar clutch, due to the arrangement of the actuating mechanism and the linkage to the other elements, and therefore likewise becomes very large in the axial direction. Furthermore, when the actuating pressure is fed in through the transmission input shaft it is necessary to seal this pressure chamber against the other pressure chambers of the converter.
One version of a power transmission device with a hydrodynamic torque converter in two-channel construction is anticipated from DE 199 09 349 A1. In this version, a lock-up clutch is assigned to the hydrodynamic rotational speed/torque converter. The piston element here is guided so that it is axially movable at the output of the power transmission unit. In addition, the piston element has a friction surface. The design of the total system is therefore characterized by a small number of components. However, this system is not suitable for applying pressure separately to the piston, in particular the actuating mechanism, since here the pressurizable piston surface bounds the pressure chamber to the hydrodynamic rotational speed/torque converter, and thus an actuation always takes place depending on the magnitude of the pressure prevailing in the hydrodynamic component.
Another version of a three-channel system for a power transmission device is anticipated from published patent DE 193 50 935 A1. Here the piston is likewise supported and guided at the connection between the input of the power transmission device and the pump wheel. The actuating mechanism is thus completely supported on the input side of the power transmission device, and is guided in the axial direction on the latter. In addition, the hydrodynamic component has a so-called pump wheel shell which is coupled to the latter in a rotationally fixed manner, which is coupled to the input of the power transmission device, forming an axial intermediate space. Integrated into this intermediate space in the axial direction are the lockup clutch and a device for damping vibrations. At the same time, the output of the LOCKUP clutch is coupled with the output of the power transmission device through the device for damping vibrations. The axial arrangement of the lockup clutch and the device for damping vibrations produces a plurality of individual pressure chambers and channels. Here too, the complete system is characterized by a significant number of components, which results not only in an enlargement of the necessary construction space in the axial direction but also in substantial additional costs.